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This Instructable describes how to make a tension scale using readily available off the shelf parts.

Materials needed:

1. Arduino - this design uses a standard Arduino Uno, other Arduino versions or clones should work also

2. HX711 on breakout board - This microchip is specially made for amplifying the signals from load cells and reporting them to another mircocontroller. The load cells plug into this board, and this board tells the Arduino what the load cells measure.

3. Tension load cell (40 kg) with brackets. There are 2 options to get one of these. You can purchase one already made (easy), or you can take apart a cheap digital luggage scale and remove the load cell (harder but you might already have one lying around). You can buy one already made with an HX711 here: https://www.amazon.com/dp/B07F922RZC

If you do purchase the kit please leave a review! It is really helpful for future buyers.

4. Mounting hardware. Depending on your application this could be rope, screws, zip ties, anything to attached the brackets to the force you want to measure.

5. Wires in various colors for connecting all the parts

6. Power supply for Arduino

Step 1: Mount the Load Cell

First we are going to mount the load cell. Your mount will be unique, but here are the guidelines you need to follow:

1. The steel load cell is a plate with strain gauges glued to the middle. The load cell measures force by sensing how much the load cell is bending.

2. Brackets attach through holes in the ends of the load cell beam. The brackets are shaped so the pulling force is applied at the center of the load cell beam. Because of their shape and the attachment location, the load cell beam bends when the brackets are pulled.

3. Hook the brackets to what you want to measure. It is best to to this with something that can move freely (like chain, hooks, strong string, or zip ties). You want the load cell and bracket assembly to be able to center itself in the direction of the load so the measurement is accurate.

Step 2: Wire the Load Cell and HX711

See the wiring diagram for how to connect the load cells, HX711, and Arduino.

On luggage style load cells like the one shown, multiple strain gauges are already wired together to for a Wheatstone bridge. All you need to do is connect the wires to the HX711 board in the correct orientation.

Step 4: Calibrate and Weigh!

The first software step is to determine calibration factors for the scale. To do this, run this code

/* Example using the SparkFun HX711 breakout board with a scale
By: Nathan Seidle
SparkFun Electronics
Date: November 19th, 2014
License: This code is public domain but you buy me a beer if you use this and we meet someday (Beerware license).
This is the calibration sketch. Use it to determine the calibration_factor that the main example uses. It also
outputs the zero_factor useful for projects that have a permanent mass on the scale in between power cycles.
Setup your scale and start the sketch WITHOUT a weight on the scale
Once readings are displayed place the weight on the scale
Press +/- or a/z to adjust the calibration_factor until the output readings match the known weight
Use this calibration_factor on the example sketch
This example assumes pounds (lbs). If you prefer kilograms, change the Serial.print(" lbs"); line to kg. The
calibration factor will be significantly different but it will be linearly related to lbs (1 lbs = 0.453592 kg).
Your calibration factor may be very positive or very negative. It all depends on the setup of your scale system
and the direction the sensors deflect from zero state
This example code uses bogde's excellent library: https://github.com/bogde/HX711

After calibrating the scale, you can run this sample program, then hack it up for your own purposes:

/* Example using the SparkFun HX711 breakout board with a scale
By: Nathan Seidle
SparkFun Electronics
Date: November 19th, 2014
License: This code is public domain but you buy me a beer if you use this and we meet someday (Beerware license).
This example demonstrates basic scale output. See the calibration sketch to get the calibration_factor for your
specific load cell setup.
This example code uses bogde's excellent library: https://github.com/bogde/HX711

bogde's library is released under a GNU GENERAL PUBLIC LICENSE
The HX711 does one thing well: read load cells. The breakout board is compatible with any wheat-stone bridge
based load cell which should allow a user to measure everything from a few grams to tens of tons.
Arduino pin 2 -> HX711 CLK
3 -> DAT
5V -> VCC
GND -> GND
The HX711 board can be powered from 2.7V to 5V so the Arduino 5V power should be fine.
*/
#include "HX711.h"
#define calibration_factor -7050.0 //This value is obtained using the SparkFun_HX711_Calibration sketch
#define DOUT 3
#define CLK 2
HX711 scale(DOUT, CLK);
void setup() {
Serial.begin(9600);
Serial.println("HX711 scale demo");
scale.set_scale(calibration_factor); //This value is obtained by using the SparkFun_HX711_Calibration sketch
scale.tare(); //Assuming there is no weight on the scale at start up, reset the scale to 0
Serial.println("Readings:");
}
void loop() {
Serial.print("Reading: ");
Serial.print(scale.get_units(), 1); //scale.get_units() returns a float
Serial.print(" lbs"); //You can change this to kg but you'll need to refactor the calibration_factor
Serial.println();
}